Fudgy Spring Brownie Bites with Fresh Raspberry Buttercream

Name: Fudgy Spring Brownie Bites with Fresh Raspberry Buttercream
Ingredients: for the fudgy brownie bites:; 1/2 c. unsalted butter (diced); 3 oz. unsweetened chocolate (chopped); 1.5 c. sugar; 3 large eggs; 1 tsp. instant espresso powder; 1.5 tsp. vanilla; 1/4 tsp. salt; 3/4 c. flour; for the fresh raspberry buttercream:; 1/2 c. unsalted butter (at room temperature); 12 oz. raspberries (fresh or frozen); 1/2 tsp. freshly squeezed lemon juice; 3.5 c. powdered sugar; pinch of kosher salt; 1 to 2 T. milk (if needed to thin out the frosting); fresh raspberries (for topping the buttercream)